Move platform specific properties on userselection to property bag #295
Description
In #287 we added a property bag to allow adding platform specific properties to the UserSelectionContext. Key-Value pairs on the property bag can be used to filter templates on the TemplatesRepositiory and as context query on composition queries.
We should gradually migrate BackendFramework, Frontendframework and Projecttype to the property bag to make CoreTS more platform agnostic.